The objective of this investigation is the elucidation of the role of membrane-bound iron-sulfur centers in electron transport and energy transduction reactions. One iron-sulfur center (the Rieske center) appears to be common to a number of membrane systems and is thought to function in the cytochrome b-c region. The relationship of this center to proton translocation mechanisms, according to the chemiosmotic hypothesis, will be examined by in situ studies as well as by studies in which are used techniques for specific removal of this carrier. The association of the Rieske center with cytochromes and quinones will be considered by isolation of electron transer complexes as well as by procedures which specifically modify or alter quinones. The function of membrane-bound iron-sulfur centers in NADH and succinic dehydrogenase of bacterial membranes will be examined by solubilization and purification of the respective enzymes. The properties of these proteins will be compared with those of analogous mitochondrial enzymes. These studies will primarily utilize electron paramagnetic resonance (EPR) spectroscopy because it is a method of proven value in the characterization of iron-sulfur centers.